A haze is an atmospheric condition marked by a slight reduction in atmospheric visibility, resulting from the formation of photochemical smog, radiation of Heat from the ground surface on hot days, or the development of a thin mist.

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Other /More definition:
A Haze is Haze consists of sufficient smoke, dust, moisture, and vapor suspended in air to impair visibility. The term regional Haze means Haze that impairs visibility in all directions over a large area.